The invention relates to light triggered & quenched static induction thyristor (referred to as LTQ SI thyristor) drive circuits, and more particularly, to a drive circuit for a LTQ SI thyristor that is turned on and off by optical triggering and quenching.
A circuit shown in FIG. 12 has been proposed as a Conventional example of this type of circuit. In FIG. 12, reference character Q1 designates a main static induction (SI) thyristor; Q2, a static induction phototransistor (SIPT) consisting of a static induction phototransistor (SIPT) Q2a and an auxiliary static induction phototransistor (SIPT) Q2b which are in Darlington connection; and R, a speed-up resistor.
The gate of main SI thyristor Q1 and the source of main SIPT Q2a of SIPT Q2 are connected to each other, the gate of main SIPT Q2a of SIPT Q2 and the source of auxiliary SIPT Q2b are connected to each other, the cathode of main SI thyristor Q1 and the gate of auxiliary SIPT Q2b are connected to each other through resistor R, and the drains of main SIPT Q2a and auxiliary SIPT Q2b are connected to each other. The anode of main SI thyristor Q1 is connected to a power supply V.sub.A through a load RL and its cathode is grounded. A drain voltage V.sub.D is applied to the mutually connected drains.
In the above construction, main SIPT Q2a and auxiliary SIPT Q2b are off and main SI thyristor Q1 is also off in the normal state. When an optically triggering photopulse LT is applied to main SI thyristor Q1, a pair consisting of electrons and holes is generated at a depletion region extending between its gate and anode and the holes are stored at the gate region. As a result, the gate voltage of main SI thyristor Q1 which has been slightly negative becomes positive, and this voltage then acts as a gate triggering voltage to forward-bias the gate region.
The static induction effect at the gate decreases a potential barrier within the channel, causing electrons to be injected from the cathode region. Electrons generated by the injected electrons and light decrease a potential barrier possessed by the holes at the anode region, thus causing the holes to be injected from the anode. When the potential at the gate region exceeds a turn-on threshold potential, main SI thyristor Q1 turns on.
Also, when an optically quenching photopulse LQ is applied to auxiliary SIPT Q2b to drive SIPT Q2 as a whole with main SI thyristor Q1 being turned on, the gate voltage of main SI thyristor Q1 drops to V.sub.D, and the holes stored at the gate region of main SI thyristor Q1 is discharged through SIPT Q2. Further, the hole current injected from the anode flows through SIPT Q2, so that main SI thyristor Q1 turns off.
In the case of the conventional drive circuit described with reference to FIG. 12, P-channel Darlington connected SIPT is used and this type of SIPT suffers from leak when the operating ambient temperature increases, and the gate of main SI thyristor Q1 is biased negatively to a great degree so that main SI thyristor cannot be turned on with small light intensity. As shown in FIG. 13, the maximum operating ambient temperature permitting the normal operation of the circuit is set to 60.degree. C. under an anode current of 10A and an anode voltage of 600 V and to as low as 25.degree. C. under an anode current of 20A and an anode voltage of 400 V when the circuit is operated at a switching frequency of 1 kHz and at a duty ratio of 50%. This disadvantageously makes the circuit inoperable under high current and high voltage when the operating ambient temperature is high.
Further, when the application of the optically quenching photopulse LQ has been ended to turn off SIPT Q2, the potential at the gate of main SI thyristor Q1 is determined by the static characteristic of SIPT Q2 in the dark state and the static characteristic of main SI thyristor Q1 in the dark state, and the gate bias at this timing rises to a level extremely close to zero. As a result, the dv/dt capability becomes so small that the circuit tends to operate erroneously.
Still further, the use of P-channel Darlington-connected SIPT suppresses I.sub.D to the order of several amperes due to restrictions in optical gain. This principally confines the optically controllable current to as low as 40A or so.
To overcome these problems, there is a circuit for quenching main SI thyristor Q1 such as shown in FIG. 14. This circuit uses a quench SI thyristor Q2c having four layers which is highly resistant to leak and a P-chancel SIT Q2d for holding quench SI thyristor Q2c in turn-off state while making the gate region of quench SI thyristor Q2c negative, so that a high operating ambient temperature and an excellent dv/dt withstand voltage characteristic can be ensured.
However, in the circuit shown in FIG. 14, quench SI thyristor Q2c must be turned off by injecting a photopulse LQb to P-channel SIT Q2d to turn P-channel SIT Q2d and negatively bias the gate of quench SI thyristor Q2c after quench SI thyristor Q2c has been turned on by receiving an optically quenching photopulse LQa at its gate region. This imposes a problem that not only LEDs (light-emitting diodes) but also an optical drive circuit, etc., must additionally be employed.